* refactor(db): simplify block height serialization
* refactor(db): make height serialization length generic
* refactor(db): create a TransactionIndex type
This changes the names of some snapshot types,
but doesn't change any data.
* refactor(db): create transparent OutputIndex and OutputLocation types
This keeps the same serialization, to avoid changing the database version.
* doc(rfc/db): make transparent database type names consistent
* doc(rfc/db): fix a bug in the Utxo.is_coinbase derivation
* fix(db): use the correct serialized size for OutputLocation